In a band of a terahertz wave, there exists a characteristic absorption band derived from the structures and states of various materials including biomolecules. Incidentally, the term “terahertz wave” employed herein refers to an electromagnetic wave having a frequency of from 30 GHz to 30 THz. An inspection technique for analyzing and identifying a material in a non-destructive manner has been developed by taking advantage of the characteristic as described above. Moreover, the application of the terahertz wave to a safe imaging technique in place of an X-ray or to a high-speed communication technology is expected.
As an analysis technique using the terahertz wave, there is a Terahertz Time Domain Spectroscopy (THz-TDS).
The THz-TDS is a measurement method for acquiring a temporal waveform of the terahertz wave (waveform of the terahertz wave represented with a time axis being taken as abscissa) which is transmitted through or reflected by a sample. A technology for acquiring physical properties of the sample by using information regarding an amplitude and a phase of the waveform acquired by this method is disclosed in Japanese Patent Application Laid-Open No. 2005-274496.
Moreover, a technology concerning an element, on which elements for generating and detecting the terahertz wave are integrated, is disclosed in Appl. Phys. Lett. 70, 2233, 1997. As an emission source of the THz wave used for the THz-TDS, a photoconductive film made of Low-Temperature grown GaAs (LT-GaAs) or the like is used. When a laser pulse beam is irradiated between electrodes on the photoconductive film, carriers instantaneously flow between the electrodes by photoexcitation. Then, a THz pulse proportional to a temporal differentiation of the carrier current is generated. Also on the detection side used for the THz-TDS, the photoconductive film is used. As is the case with the emission source, the laser pulse corresponding to an ultrashort pulse is used.
As described above, the Terahertz time domain spectroscopy is carried out by using the ultrashort pulse having a time width smaller than that of the terahertz wave. Specifically, a pulse laser having a pulse width of several tens of femtoseconds is used as the ultrashort pulse for sampling an amplitude (such as a photocurrent value) at a certain time on the temporal waveform of the terahertz wave. Then, the timing of irradiating light to the position at which the terahertz wave is emitted or detected is changed. As a result, the amplitude (such as the photocurrent value) of the terahertz wave can be acquired while the position on the temporal waveform at which each sampling is performed is being changed. As a result, the whole temporal waveform can be reproduced.
In order to change the timing of irradiation, a movable mirror for changing an optical path length of the ultrashort pulse is used as an optical delay system.
As described above, in order to acquire the temporal waveform of a terahertz wave, a method of using an optical delay system to change the timing of irradiating light to the position at which the terahertz wave is emitted or detected is known.
The present inventors have conducted extensive studies on a technology of acquiring a temporal waveform of a terahertz wave by using a technique different from the method described above, and have found that a temporal waveform can be reproduced even without using an optical delay system and accomplished the present invention.